Sometimes it is not enough to know whether an element is in the slice. One may want to insert a new element into the slice. But without knowing what is in the slice already. So, a slightly tweaked search will find the position where it should go.

sortedInsert.go

package main

import (
	"fmt"
	"sort"
)

/*
	Sometimes it is not enough to know whether an element is in the slice. One
	may want to insert a new element into the slice. But without knowing what
	is in the slice already. So, a slightly tweaked search will find the
	position where it should go.

	Obviously, checking whether the element is smaller than the first, and is
	larger than the last will help to find the `edge` scenarios. But for a
	generic case see the code below

	-- (sillyfellow@05/22/2020) */

// findIndex to insert helps you to insert a new element in the slice without
// breaking the sorted order. For the documented usage of sort.Search, see:
// https://play.golang.org/p/dl4mu0HWJty
func sortedInsert(sortedSlice []int, x int) {
	i := sort.Search(len(sortedSlice), func(i int) bool { return sortedSlice[i] > x })
	if i == 0 {
		fmt.Printf("%d should be in the beginning\n", x)
		fmt.Println("sortedSlice = append([]int{x}, sortedSlice...)\n")
	} else if i < len(sortedSlice) {
		if sortedSlice[i-1] != x {
			fmt.Printf("%d should go at %d (before %d in %v)\n", x, i, sortedSlice[i], sortedSlice)
			fmt.Println("sortedSlice = append(sortedSlice[:i], append([]int{x}, sortedSlice[i:]...)...)\n")
		} else {
			fmt.Printf("%d is already at %d (before %d in %v)\n", x, i-1, sortedSlice[i], sortedSlice)
			fmt.Println("nothing to do\n")
		}
	} else {
		fmt.Printf("%d should go at the end (index: %d), after %d\n", x, i, sortedSlice[i-1])
		fmt.Println("sortedSlice = append(sortedSlice, x)\n")
	}
}

func main() {

	sortedSlice := []int{1, 3, 6, 10, 15, 21, 28, 36, 45, 55}

	sortedInsert(sortedSlice, 0)
	//0 should be in the beginning
	//sortedSlice = append([]int{x}, sortedSlice...)

	sortedInsert(sortedSlice, 1)
	//1 is already at 0 (before 3 in [1 3 6 10 15 21 28 36 45 55])
	//nothing to do

	sortedInsert(sortedSlice, 2)
	//2 should go at 1 (before 3 in [1 3 6 10 15 21 28 36 45 55])
	//sortedSlice = append(sortedSlice[:i], append([]int{x}, sortedSlice[i:]...)...)

	sortedInsert(sortedSlice, 3)
	//3 is already at 1 (before 6 in [1 3 6 10 15 21 28 36 45 55])
	//nothing to do

	sortedInsert(sortedSlice, 5)
	//5 should go at 2 (before 6 in [1 3 6 10 15 21 28 36 45 55])
	//sortedSlice = append(sortedSlice[:i], append([]int{x}, sortedSlice[i:]...)...)

	sortedInsert(sortedSlice, 54)
	//54 should go at 9 (before 55 in [1 3 6 10 15 21 28 36 45 55])
	//sortedSlice = append(sortedSlice[:i], append([]int{x}, sortedSlice[i:]...)...)

	sortedInsert(sortedSlice, 55)
	//55 should go at the end (index: 10), after 55
	//sortedSlice = append(sortedSlice, x)

	sortedInsert(sortedSlice, 57)
	//57 should go at the end (index: 10), after 55
	//sortedSlice = append(sortedSlice, x)
}